This invention relates to a solution for coating plastic material such that it can be tinted and the process for tinting plastic material and more specifically, for tinting polycarbonate and other plastic eyeglass lenses.
Presently eyeglass lenses are made from either glass or plastic. Plastic lenses have advantages over glass in that they are lighter and can be easily tinted by an optical laboratory using procedures well known in the art. The major drawback to most currently used tintable plastic lenses is that they are easily scratched and broken upon impact.
Polycarbonate is a synthetic thermoplastic resin with characteristics which make it a better material for use in eyeglass lenses than glass and allyl diglycol carbonate (CR-39 made by PPG, Pittsburgh, PA), the leading plastic used for plastic lenses today. Polycarbonate is lighter than glass and CR-39, making optical lenses made from this material more comfortable in that there is less strain due to the weight of the lenses on a wearer's nose and ears. Polycarbonate is also less expensive to manufacture than glass and CR-39 and has a higher refractive index and substantially the same light transmittance as glass. Both polycarbonate and CR-39 lense are generally coated with a material that imparts a scratch resistance to these plastics such as silica-silanol. Most significantly, coated polycarbonate has equal or greater scratch resistance than CR-39 and significantly greater impact resistance. The one drawback to coated polycarbonate is the inability of dye to penetrate coatings applied to polycarbonate lenses to tint them.
The lens manufacturer generally produces semifinished CR-39 plastic lens blanks with the convex surface cut, ground and polished. The lens is also coated with a hard scratch resistant coating that inhibits absorption of the tinting dye through that surface. An optical laboratory receiving the semifinished lens, cuts, grinds and polishes the concave surface of the lens to a specific prescription thereby grinding the coating off and then tints it to a customer's specifications. The uncoated surface of the CR-39 lens absorbs the dye during the tinting process. The lens is then edged to fit the eyeglass frame. Polycarbonate finished lenses, on the other hand, are generally produced in a single molding step for a preselected prescription by the manufacturer, thereby making grinding and polishing steps unnecessary, and are then coated with a scratch resistant coating. The optical laboratories, in general, do not have the tools to cut and grind to a specified prescription lenses made of polycarbonate. Polycarbonate lenses, because of the presence of the coating, cannot be tinted by the standard techniques used in the industry.
Uncoated polycarbonate material may be tinted with dyes typically used for tinting plastic lenses if the tinting solution is heated to 190.degree. to 195.degree. F. However, uncoated polycarbonate lenses are not generally distributed by the manufacturers because of the poor scratch resistant qualities of the uncoated material. The present procedure for tinting uncoated polycarbonate lenses is by vacuum deposition.
Under current methodology, CR-39 plastic lenses are tinted by the following procedure. After finishing, the lenses are dipped in a dye solution and slowly removed. The duration the lens is submerged in the dye solution determines the degree to which the lens is tinted. In a last step which is performed by some optical laboratories, the lens is coated providing greater scratch resistance. Most of the optical laboratories are set up to perform the above-described tinting procedure.
The present invention teaches a composition and method useful in preparing polycarbonate such that it can be tinted using similar techniques and the same equipment and dyes as those currently being used in the industry for CR-39 lenses.